Time-stretch analog-to-digital converter

It relies on the process of time-stretch, which effectively slows down the analog signal in time (or compresses its bandwidth) before it can be digitized by a standard electronic ADC.

There is a huge demand for very high-speed analog-to-digital converters (ADCs), as they are needed for test and measurement equipment in laboratories and in high speed data communications systems.

Finally, this obtained optical pulse is converted to the electrical domain by a photodetector, giving the stretched replica of the original analog signal.

The PTS processor is based on specialized analog optical (or microwave photonic) fiber links[5] such as those used in cable TV distribution.

In a conventional analog optical link, dispersion causes the upper and lower modulation sidebands, foptical ± felectrical, to slip in relative phase.

The time-lens concept relies on the mathematical equivalence between spatial diffraction and temporal dispersion, the so-called space-time duality.

Theoretically, a focused aberration-free image is obtained under a specific condition when the two dispersive elements and the phase shift satisfy the temporal equivalent of the classic lens equation.

[10] In contrast to the time-lens approach, PTS is not based on the space-time duality – there is no lens equation that needs to be satisfied to obtain an error-free slowed-down version of the input waveform.

Another important difference between the two techniques is that the time lens requires the input signal to be subjected to high amount of dispersion before further processing.

In addition to wideband A/D conversion, photonic time-stretch (PTS) is also an enabling technology for high-throughput real-time instrumentation such as imaging[11] and spectroscopy.

Wavelength-time spectroscopy, which also relies on photonic time-stretch technique, permits real-time single-shot measurements of rapidly evolving or fluctuating spectra.

Each pulse representing one frame of the camera is then stretched in time so that it can be digitized in real-time by an electronic analog-to-digital converter (ADC).

The ultra-fast pulse illumination freezes the motion of high-speed cells or particles in flow to achieve blur-free imaging.

A time-stretch analog-to-digital converter (with a stretch factor of 4) is shown. The original analog signal is time-stretched and segmented with the help of a time-stretch preprocessor (generally on optical frontend ). Slowed down segments are captured by conventional electronic ADCs. The digitized samples are rearranged to obtain the digital representation of the original signal.
Optical frontend for a time-stretch analog-to-digital converter is shown. The original analog signal is modulated over a chirped optical pulse (obtained by dispersing an ultra-short supercontinuum pulse from a mode-locked laser, MLL). Second dispersive medium stretches the optical pulse further. At the photodetector (PD) output, stretched replica of original signal is obtained.
Capture of a 95 GHz RF tone using the photonic time-stretch digitizer. The signal is captured at an effective sample rate of 10 terasamples per second.